The invention relates to a device for monitoring the supply of electrically conductive recording fluid in a supply container for ink recording devices and other recording devices which employ recording fluid, utilizing electrodes which are wetted by the recording fluid and a circuit arrangement which detects the electrical resistance between the electrodes and actuates a suitable display device when the amount of recording fluid in the container falls below a specific volume.
In ink recording devices of the type involved, generally a recording head is moved by suitable motor drive means along a data carrier and supplied, over a supply line, with recording fluid from an ink supply container. If the ink supply container is to be integrated in the recording head, it is not possible to visually monitor the ink supply therein. However, a constant monitoring of the ink supply is necessary, in particular, in recording devices in the teleprinter and data communications field.
IBM Technical Disclosure Bulletin Vol. 16, No. 3, of August 1973, on page 775, discloses a device for determining the ink level in an open vessel. Such device utilizes two electrodes which are submerged into the fluid, with the capacitance between the electrodes being measured by means of a suitable circuit arrangement. Such capacitance changes in accordance with the level of the recording fluid and thus may be employed as a criterion therefor.
A measuring operation of this type is not suitable when an electrically conductive recording fluid is employed in the ink recording device, and in addition the arrangement of the electrodes necessitates an open supply vessel. Ink supply containers which are sealed by an elastic membrane and change their supply volume in accordance with the consumption of recording fluid, cannot be used.
In an effort to avoid these disadvantages, it has previously been proposed that, instead of measuring the capacitance by means of two electrodes, an electrically conductive recording fluid be employed, and the electrical resistance between the two electrodes, which changes during the consumption of recording fluid, be measured and, upon the amount of recording fluid in the container falling below a specific supply volume, actuate a warning device.
The ohmic resistance of electrically conductive recording fluid utilized in such recording devices is, however, strongly temperature-dependent. Consequently, it is necessary to compensate for temperature changes, and it has previously been proposed that this should be effected over a temperature dependent resistance. Irrespective thereof, in a circuit arrangement of this type, the point of disconnection remains dependent upon the production-related fluctuation in the ink conductivity.